fix: honour PR_SPLIT_* env vars for strategy and priority options

[vitali87]

Summary

The README's Configuration table documents PR_SPLIT_PRIORITY, PR_SPLIT_CHUNK_STRATEGY and PR_SPLIT_PARTITION_STRATEGY, but the split options had no envvar= binding. Typer passed the explicit default into Settings(...), which overrides pydantic-settings' env lookup, so the variables were silently ignored (verified: PR_SPLIT_PARTITION_STRATEGY=graph still produced llm).

• Bind envvar on --priority, --chunk-strategy, --partition-strategy, and --cp-sat-timeout (PR_SPLIT_CP_SAT_TIMEOUT)
• Document --cp-sat-timeout / PR_SPLIT_CP_SAT_TIMEOUT, which were missing from both README tables

Test plan

• New test_split_uses_strategy_envvars covering all four variables
• uv run ruff check / ruff format --check clean
• uv run pytest -q — 445 passed

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between db793ec and 1221e85.

📒 Files selected for processing (3)

• README.md
• pr_split/cli.py
• tests/test_cli_new_features.py

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[greptile-apps]

Greptile Summary

This change makes the documented PR_SPLIT_PRIORITY, PR_SPLIT_CHUNK_STRATEGY, PR_SPLIT_PARTITION_STRATEGY, and PR_SPLIT_CP_SAT_TIMEOUT settings effective for pr-split split, and documents the CP-SAT timeout setting. The environment-binding failure hypothesis was disproved by exercising split --dry-run with all four values and confirming that correctly parsed settings reach planning; explicit command-line options also override environment values.

Confidence Score: 5/5

Safe to merge: the changed CLI path correctly applies the documented environment settings while preserving explicit command-line overrides.

The command was exercised with all four environment values, enum and float conversion was confirmed at the planner boundary, explicit option precedence was checked, and focused CLI tests passed.

Files Needing Attention: No files need further attention.

T-Rex Logs

What T-Rex did

• Reviewed the general-contract-validation-proof to compare pre-change default environment bindings with post-change explicit options for the split command.
• Validated the env-bindings behavior by running the env-bindings validation script, which exercises the Typer command and mocks external boundaries after parsing.
• Inspected the before-and-after command output logs to confirm the exact values produced by the split command across the change boundary.
• Checked the targeted tests log to verify test coverage for the new env-bindings validation behavior.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "fix: honour PR_SPLIT_* env vars for stra..." | Re-trigger Greptile}